JP5492379B2 - Method of receiving session key in home network and method of reproducing content using the same - Google Patents

Description

  The present invention relates to a method for receiving a session key in a home network and a method for reproducing content using the same, and more particularly, to receive a session key that can more easily manage domain members using a session version. The present invention relates to a method and a content reproduction method using the method.

  Digital content is transmitted from the content provider to the user. Content must be protected so that digital content can only be used by users who have gained legitimate rights through payment of costs for the content, and digital content cannot be used by users who do not have legitimate rights. .

  In order to prevent acquisition of content for users without legitimate authority, the content is encrypted with a content key, and the content key is distributed only to users with legitimate authority.

  On the other hand, with the recent development of home network technology, one user owns one or more user devices, and contents can be moved between them. The user desires to use the content for all the devices owned by the user with one cost settlement. However, if the content can be freely moved in a form that can be reproduced between devices, an unauthorized user can acquire and use the content. Therefore, in the home network technology, content can be transferred between user devices in the home network of an authorized user, and an unauthorized user cannot acquire content even if the content cannot be acquired. Technology is needed to make it unusable.

  In particular, the Federal Communications Commission (FCC) has issued a 1-bit broadcast flag (BF) for high-definition HD-class content broadcast in digital broadcasting in the United States from July 2005. When the BF of the corresponding content is 1, the technology to prevent the use of unauthorized users is required in the digital broadcasting implementation technology standard so that the content is protected, that is, the home server. The demand for the safe use of digital broadcast content between devices and user equipment is even more urgent.

  FIG. 1 is a diagram illustrating a structure of a home network (HN).

  The content provider CP transmits the content to a home server (HS: Home Server) through the transmission channel 10.

  The HS is connected to the user devices D1, D2, D3, and D4, manages the subscription and withdrawal of the legitimate user equipment, and transmits the content only to the legitimate authorized user equipment, that is, the currently joined equipment. .

  Here, the domain means a set of user equipment connected to one HS. The domain always changes as the user equipment joins and leaves the HN. That is, the user equipment connected to the domain always changes. Therefore, the withdrawn user equipment needs a process for making it impossible to acquire the content any more, which is a domain management method using a session key.

  FIG. 2 is a diagram illustrating a conventional domain management method.

  In step 210, the HS uses the predetermined content key K_cont to encrypt the content Cont and the encrypted content E K_cont, Cont and the content described above using the predetermined session key K_sess. The encrypted content keys EK_sess and K_cont generated by encrypting the key K_cont are all transmitted to the current user devices D1, D2, D3 and D4. Here, the current user device means a device that is currently joined and connected to the HS with a legitimate authority. Generally, the encrypted content keys EK_sess, K_cont are stored in each user device in the form of a file called a license.

  In step 220, the HS transmits the session key K_sess used for encrypting the content key to the user devices D1, D2, D3, and D4 through a secure transmission path.

  In step 230, the user devices D1, D2, D3, D4 obtain the content key K_cont by decrypting the encrypted content keys EK_sess, K_cont using the session key K_sess, and obtain the content key K_cont. After acquiring the content Cont by decrypting the encrypted content EK_cont, Cont described above using the, the content can be reproduced.

  In step 240, assume that user D4 has left.

  In step 250, the HS generates a new session key K_sess' and transmits it to the current user equipment D1, D2, D3.

  In step 260, the user equipment D1, D2, D3 uses the new session key K_sess' received in step 250 to encrypt the content key K_cont of step 230, thereby creating a new encrypted content key. After generating EK_sess' and K_cont, save them in the license file. That is, the user devices D1, D2, and D3 update the license file.

  In step 270, the user devices D1, D2, and D3 obtain the content key K_cont by decrypting the encrypted content keys EK_sess 'and K_cont using the session key K_sess' and obtain the content key K_cont. After the content Cont is acquired by decrypting the encrypted content EK_cont, Cont described above using the above, it can be reproduced.

  For new content thereafter, new content Cont 'is transmitted to the user devices D1, D2, D3 using the new session key K_sess'. In other words, the HS encrypts the new content Cont 'using a predetermined new content key K_cont', thereby generating the encrypted content E K_cont ', Cont' and the new content described above. By encrypting the content key K_cont ′ described above using the session key K_sess ′, the generated encrypted content keys EK_sess ′ and K_cont ′ are all transmitted to the current user equipment D1, D2, and D3. .

  On the other hand, since the user device D4 has a new session key K_sess', the content key K_cont 'for the newly transmitted content Cont' cannot be acquired, so that the user device D4 is distributed in the HN after the withdrawal of the user device D4. Content that cannot be played back.

  However, according to such a conventional method, the user device D4 cannot transmit the content Cont transmitted to the user device before the withdrawal of the user device D4 from the user devices D1, D2, D3. Have.

  If the user device D4 does not have the content key K_cont that the user device D4 has, the user device D4 receives the license file including the encrypted content keys EK_sess, K_cont, Although the content key K_cont must be acquired, since the user devices D1, D2, and D3 have updated their licenses as in step 260 after the withdrawal of the user device D4, the content key encrypted with the previous session key K_sess E_K_sess and K_cont are not held, and eventually the user device D4 cannot acquire the content key K_cont.

  That is, when the user device D4 loses the content E K_cont, Cont and the content key K_cont, the content Cont is the content before the withdrawal of the user device D4. There is a problem that even if K_cont and Cont are transmitted from the user devices D1, D2 and D3, they cannot be used.

  The present invention has been devised to solve the above-described problems, and can perform various domain management such as allowing content transmission between user devices freely using a session key and a session version. It is an object of the present invention to provide a session key receiving method and a content reproducing method using the same.

  In order to achieve the above object, the present invention provides a method of receiving a session key from an HS in an HN including a plurality of members. A) Each time the member of the HN changes, the session key and the session key Receiving and storing a session version representing a generation order of the content; b) receiving a license necessary for playing a predetermined content; c) an encrypted session version extracted from the license and the storage Determining a session key necessary for playing the content based on the session version that has been made.

  Here, the license includes a server identifier that is an identifier of the HS, and before the c) step, d) the server identifier included in the license is the server of the HS to which the user device that receives the session key belongs. The method further includes the step of determining a session key necessary for reproducing the content by determining whether the identifier is an identifier.

  Here, the steps c) and d) are performed based on a current session key table (CSKT), and the CSKT includes a current HS server identifier, a current session version, and a current session key.

  Here, the license includes an encrypted content key generated by encrypting the content key using the server identifier, the session version, and the session key, and the step c) includes the step of This is done by comparing the session version included in the license with the session version included in the CSKT.

  Here, the license includes an encrypted content key generated by encrypting the content key using the server identifier, the session version, and the session key, and d) step. Is performed by determining whether or not the server identifier included in the license is included in the server identifier included in the CSKT.

  Here, the steps c) and d) are performed based on a past session key table (PSKT), and the PSKT includes a server identifier of a past home server, a past session version, and a past session key, respectively. Contains at least one or more.

  Here, the step c) includes the steps c1) determining the current session key as the session key when the session version from the license matches the current session version, and c2) the session from the license. If the version is smaller than the current session version, it checks whether a session version from the license exists in the PSKT, and if it exists, determines the existing past session key as the session key. Steps.

  Further, the step d) ends when the server identifier from the license does not match the server identifier in the CSKT by making a reproduction prohibition decision for the content.

  The present invention also provides: e) If there is no session key necessary for playback of the content among the stored session keys, the latest session key and the session version of the session key are obtained from the HS in the HN. The method further includes updating the session key by receiving.

  Further, the present invention provides a method in which a second user device transmits a predetermined content from a first user device and reproduces it in an HN having a plurality of user devices, and a) whenever a change occurs in the user device of the HN. Receiving and storing a session key necessary for reproducing the content and a session version representing the generation order of the session key; and b) encrypted using a predetermined content key from the first user device. Receiving a content and a license required to play the content; c) determining a session key required to play the content based on a session version included in the license and the stored session version. And d) the content using the determined session key. Generate the key, including the steps of reproducing the encrypted content using the content key that is generated.

  According to the present invention, the home server transmits a session key and a session version to the user equipment every time a subscriber change occurs, and the user equipment determines the session key using the session version, Various domain management is possible, such as free transmission of content between user devices.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 3 is a diagram illustrating a method for receiving a session key according to the first embodiment of the present invention.

  At step 310, a change in HN membership occurs due to user equipment subscription or withdrawal.

  In step 320, each time a change occurs in a member of the HN, the user equipment receives and stores a session version representing the session key and the session key generation order from the HS.

  If the user equipment D1, D2, D3, D4 has acquired the content key K_cont using the session key K_sess whose session version is 1, and then the user equipment D4 leaves HN, the HS The new session key K_sess ′ and the session version = 0002 of this session key K_sess ′ are transmitted to the current user equipment D1, D2, D3. That is, the user devices D1, D2, and D3 receive the session key Kess' and the session version = 0002 from the HS and store them.

  In one embodiment, the user equipment stores a session key and a session version in the CSKT, and the CSKT has a format as shown in Table 1, for example.

  Table 1 shows that the current session key is 2222 and the session version is updated from 0001 to 0002.

  In step 330, the user devices D1, D2, and D3 receive the license Lic necessary for reproduction of the predetermined content, and extract the encrypted session version ESV from the received license Lic. The encrypted session version (ESV: Encoding Session Version) is a number representing the generation order of session keys used by the HS to encrypt the content key used for decrypting the corresponding content. That is, in the present invention, the license Lic is configured as shown in Table 2.

  Here, the license Lic means a license used for reproducing the content Cont, the content key K_cont is a content key used for encrypting the content Cont, and the session key K_sess is used for encrypting the content key K_cont. The session key used.

  In step 340, the user equipments D1, D2, and D3 check whether the ESV extracted in step 330 and the current session version (CSV: Current Session Version) in the CSKT are the same. If they are the same, proceed to 350; otherwise, proceed to step 360.

  In step 350, the user equipment D1, D2, D3 extracts CSK corresponding to the CSV as a session key.

  In step 360, the user devices D1, D2, and D3 request the session key corresponding to the ESV from the HS, and receive the desired session key through user authentication.

  FIG. 4 is a diagram illustrating a method in which the withdrawn user D4 transmits and plays back the content Cont_A from the user device D1 using the session key received by the method of FIG.

  The user devices that have joined the HS HN at time t1 are user devices D1, D2, D3, and D4, which are using the content Cont_A. At time t2, the user equipment D4 has left. According to the method of FIG. 3, after time t2, the HS distributes the content Cont_B to the user devices D1, D2, D3 using the new session key K_sess'. This is summarized as follows.

  Since the user devices D1, D2, and D3 are currently receiving and using the content Cont_A, they have the encrypted content EK_cont, Cont_A, and the license Lic_A. However, since the session key is updated by the withdrawal of the user equipment D4, the CSKTs possessed by the user equipment D1, D2, D3 after time t2 are as follows.

  Under such circumstances, due to reasons such as loss, the user device D4 tries to use the content Cont_A transmitted from the user device D1 after withdrawal.

  In step 410, the user device D4 receives the encrypted contents EK_cont, Cont_A and the license Lic_A from the user device D1.

  Unlike the conventional method, since the license is not updated even if a change occurs in the HN member, the license Lic_A transmitted from the user device D1 is set to ESV = 0001 and encrypted content keys E K_sess, K_cont.

  In step 420, the user equipment D4 extracts the ESV from the license Lic_A, and compares the ESV with the CSV in the CSKT of the user equipment D4. If they are the same, proceed to step 430; otherwise, proceed to step 460.

  Since ESV = 0001 and CSV is also 0001, it can be seen that the content key K_cont used for reproducing the content Cont_A is encrypted with the same session key K_sess.

  In step 430, the user device D4 extracts the encrypted content keys EK_sess, K_cont from the license Lic_A received in step 410.

  In step 440, the user equipment D4 generates a content key K_cont by decrypting the encrypted content keys E K_sess and K_cont extracted in step 430 using CSK = K_sess of CSKT.

  In step 450, the user device D4 uses the content key K_cont generated in step 440 to extract and extract the content Cont_A by decrypting the encrypted content EK_cont and Cont_A received in step 410. Played content Cont_A is reproduced.

  In step 460, the user equipment D4 requests a session key corresponding to the ESV from the HS, and receives a desired session key through user authentication.

  In step 470, the user equipment D4 generates a content key by using the session key received in step 460 to decrypt the encrypted content key in the license Lic_A, and uses the generated content key. Then, the content Cont_A is reproduced.

  According to the method of FIG. 4, the license included in the user equipment is not updated even if there is a change in subscriber, so in step 420 the CSV and ESV are always the same. Therefore, unless exceptional cases occur, steps 430 to 450 are performed, and steps 460 to 470 do not occur. That is, the withdrawn user equipment D4 can still use the content before withdrawal from another user equipment D1.

  FIG. 5 is a diagram illustrating a method for receiving a session key according to the second embodiment of the present invention.

  According to the session key receiving method of FIG. 3, the user equipment that has withdrawn still has the advantage that the content before the withdrawal can be transmitted and used from other user equipment, The content before the withdrawal is transmitted from the user equipment to which the user is subscribed and cannot be reproduced.

For example, in the case of FIG. 4, the user device D4 can transmit the content Cont_A from the user device D1 and can reproduce it, but even if the user device D2 transmits the content Cont_A from the user device D1, the user device D2 The content Cont_A cannot be reproduced because it has the updated session key K_sess' and does not have the past session key K_sess . The session key reception method of FIG. 5 provides a solution to this problem.

  At step 510, a change in HN membership occurs due to a user equipment subscription or withdrawal.

  In step 520, each time a change occurs in a member of the HN, the user equipment receives and stores a session version representing the session key and the session key generation order from the HS.

At this time, the user equipment stores the CSK and CSV in the CSKT, and stores the PSK (PSK: Previous Session Key) and the past session version (PSV: Previous Session Version) in the PSKT. CSKT and PSKT have, for example, the formats shown in Table 5 and Table 6, respectively.

  In step 530, the user devices D1, D2, and D3 receive the license Lic necessary for reproducing the predetermined content, and extract the ESV from the received license Lic.

  At time t2, the user device D4 withdraws, the content before the withdrawal of the user device D4 is content Cont_A, the content after the withdrawal is Cont_B, and the session keys distributed to the user devices at this time are K_sess and K_sess' The situation can be summarized as in Table 7 below.

  At this time, the license Lic has the configuration shown in Table 8 as in Table 2.

  Here, the license Lic means a license used for reproducing the content Cont, the content key K_cont is a content key used for encrypting the content Cont, and the session key K_sess is used for encrypting the content key K_cont. The session key used.

  In step 540, the user equipment D1, D2, D3 checks whether the ESV extracted in step 520 is the same as the CSV in the CSKT. If yes, go to step 550, otherwise go to 555.

  In step 550, the user devices D1, D2, and D3 extract CSK corresponding to the CSV as a session key.

In step 555, the ESV and CSV are compared . If the ESV is smaller than the CSV, go to step 560; otherwise, go to step 580.

  In step 560, ie, if the ESV is less than the CSV, the user equipment D1, D2, D3 checks whether there is an ESV in the PSKT. If it exists, go to 570; otherwise, go to step 580.

  In step 570, the user equipment D1, D2, D3 extracts a session key corresponding to the ESV from the PSKT.

  If it is determined in step 580 that the ESV is greater than the CSV or the corresponding ESV does not exist in the PSKT in step 560, the user equipment D1, D2, and D3 are in the session corresponding to the ESV in the HS. Request a key and receive the desired session key through user authentication.

  FIG. 6 is a diagram illustrating a method in which the user D3 transmits the content Cont_A from the user device D1 and reproduces it using the session key received by the method of FIG. 5 after the user device D4 withdraws.

  The user devices that have joined the HS HN at time t1 are user devices D1, D2, D3, and D4, which are using the content Cont_A. At time t2, the user equipment D4 has left. This situation is summarized as Table 7 described above.

  Since the user devices D1, D2, and D3 are currently receiving and using the content Cont_A, they have the encrypted content EK_cont, Cont_A, and the license Lic_A. However, since the session key is updated by the withdrawal of the user device D4, the CSKT and PSKT that the user devices D1, D2, and D3 have after the time t2 are as shown in Tables 5 and 6, respectively.

  In such a situation, due to reasons such as loss, the user device D3 attempts to transmit and use the content Cont_A from the user device D1 after the user device D4 leaves.

  In step 610, the user device D3 receives the encrypted content EK_cont, Cont_A and license Lic_A from the user device D1.

  As shown in Table 7, the license Lic_A transmitted from the user device D1 includes ESVESV = 0003 and encrypted content keys EK_sess, K_cont.

  In step 620, the user equipment D3 extracts the ESV from the license Lic_A, and compares the ESV with the CSV in the CSKT of the user equipment D3. If ESV and CSV are the same, go to step 630; otherwise, go to step 640 or 650.

  In step 630, the user equipment D3 extracts CSK corresponding to the CSV as a session key.

  In step 640, if the ESV is greater than the CSV, the user equipment D3 requests the session key corresponding to the ESV from the HS, and receives the desired session key through user authentication.

  In step 650, if the ESV is smaller than the CSV, the user equipment D3 checks whether there is an ESV in the PSKT. If yes, go to Step 660; otherwise, go to Step 670.

  In step 660, the user equipment D3 extracts a session key corresponding to the ESV from the PSKT.

  In step 670, the user equipment D3 requests a session key corresponding to the ESV from the HS, and receives a desired session key through user authentication.

  In step 680, the user equipment D3 uses the session key received in step 640 or 670 or the session key extracted in step 630 or 660, and the encrypted content key E K_sess in the license Lic_A received in step 610. , K_cont is decrypted to generate a content key K_cont.

  In step 690, the user equipment D3 generates content by decrypting the encrypted content using the content key generated in step 680, and reproduces the content.

  FIG. 7 is a diagram showing the structure of HN when HS is 2 or more.

  In HN, there may be more than one HS. Furthermore, one user device may belong to two or more HSs. FIG. 7 is a drawing showing an example of such a case. In FIG. 7, the user equipments D1, D2, D3, D4 connected to the first home server HS1 and the first home server HS1 constitute the first home network HN1, and are connected to the second home server HS2 and the second home server HS2. It is also possible for the user devices D2 and D3 thus made to form the second home network HN2. At time t2, the user equipment D4 tries to leave the first home network HN1.

  In such a case, the session key receiving method of FIGS. 3 and 5 cannot distinguish between two or more session keys generated by each home server. The embodiment of FIG. 8 presents a solution to such a problem.

  FIG. 8 illustrates a session key receiving method according to the third embodiment of the present invention.

  At step 810, a change in home network member occurs due to a user equipment subscription or withdrawal.

  In step 820, each time a change occurs in a member of the home network, the user equipment changes the session key, the session version indicating the session key generation order, and the server identifier SUID that is the identifier of the home server that generated the corresponding session key. Receive and save from one or more home servers.

At this time, the user equipment stores the server identifier, the current session key, and the CSV in CSKT, and stores the server identifier, PSK, and PSV in PSKT.

  As shown in FIG. 7, since the user devices D2 and D3 are currently subscribed to the first home network and the second home network at the same time, the CSKT and PSKT of the user devices D2 and D3 are as shown in Table 9 and Table 10, respectively. Street.

  Further, as shown in FIG. 7, since the user equipment D1 is currently subscribed only to the first home network, the CSKT and PSKT of the user equipment D1 are as shown in Table 11 and Table 12, respectively.

  In the home network belonging to the first home server HS1, the user device D4 withdraws at time t2, and the content before the withdrawal of the user device D4 is the content Cont_A, and the content after the withdrawal is Cont_B. If the session keys are K_sess and K_sess', the current situation can be summarized as shown in Table 13 below.

  The license Lic_A for the content before withdrawal of the user device D4 has a configuration as shown in Table 14.

  The license Lic_B for the content after withdrawal of the user device D4 has a configuration as shown in Table 15.

  On the other hand, in the home network belonging to the second home server HS2, there is no change in user equipment at time t2. Accordingly, the situation associated with the second home server can be summarized as shown in Table 16 below.

  Here, the contents Cont_C and Cont_D and the licenses Lic_C and Lic_D mean the contents transmitted by the second home server HS2 to the user devices D2 and D3 and the respective licenses.

  The licenses Lic_C and Lic_D have configurations as shown in Table 17 and Table 18, respectively.

  Here, K_cont_C and K_cont_D represent content keys used for encrypting the content Cont_C and Cont_D, respectively. The two content keys may or may not be the same.

  In step 830, the user device receives a license Lic necessary for reproducing predetermined content, and extracts a server identifier SUID from the received license Lic.

  In step 835, the user equipment checks whether the server identifier SUID extracted in step 830 exists in its own CSKT. If it exists, the process proceeds to step 840, and if it does not exist, the process proceeds to step 837.

  In step 840, if the CSKT includes the SUID, the user equipment extracts the ESV from the license Lic, and checks whether the extracted ESV and the CSV in the CSKT are the same. If they are the same, proceed to step 850, otherwise proceed to step 855.

  In step 850, the user equipment extracts a CSK corresponding to the CSV as a session key.

In step 855, the ESV and CSV are compared . If the ESV is smaller than the CSV, go to step 860; otherwise, go to step 880.

  In step 860, i.e., if the ESV is less than the CSV, the user equipment checks whether there is an ESV in the PSKT. If yes, go to Step 870; otherwise, go to Step 880.

  In step 870, the user equipment extracts a session key corresponding to the ESV from the PSKT.

  If it is determined in step 880 that the ESV is larger than the CSV or the corresponding ESV does not exist in the PSKT in step 860, the user equipment requests a session key corresponding to the ESV from the HS, A desired session key is received through user authentication.

  In step 837, if the CSKT does not include the SUID, the user equipment displays a message to the user that playback is not possible.

  According to the session key receiving method of FIG. 8, the server identifier for identifying the home server exists in the license transmitted from the home server, and the server identifier also exists in the CSKT or PSKT of the user equipment. . Accordingly, since the user device can determine whether or not the session key is for the content transmitted from the home server to which the user device belongs, the present invention is applied to a home server in which one user device is connected to a plurality of home servers. Make applicable.

  On the other hand, the session key receiving method according to the present invention can be created by a computer program. The code and code segments that make up the program can be easily inferred by computer programmers in the field. Further, the program is stored in a computer-readable information recording medium, and is read and executed by the computer, thereby realizing a session key receiving method. The information recording medium includes a magnetic recording medium, an optical recording medium, and a carrier wave medium.

  In the above, this invention was demonstrated centering on the desirable embodiment. Those skilled in the art will appreciate that the present invention can be embodied in a modified form without departing from the essential characteristics of the present invention. Accordingly, the disclosed embodiments should be considered in an illustrative, not a limiting sense. The scope of the present invention is shown not in the above description but in the claims, and all differences within the scope equivalent thereto should be construed as being included in the present invention.

It is drawing which shows the structure of a home server. 2 is a diagram illustrating a conventional domain management method. 3 is a diagram illustrating a method for receiving a session key according to the first embodiment of the present invention. 4 is a diagram illustrating a method in which a user who has withdrawn transmits and reproduces content before withdrawal from another user device using a session key received by the method of FIG. 3. 6 is a diagram illustrating a method for receiving a session key according to a second embodiment of the present invention. 6 is a diagram illustrating a method in which another user D3 transmits and plays back content from another user device using the session key received by the method of FIG. 5 after withdrawal of the user device. It is drawing which shows the structure of a home network in case a home server is two or more. 6 is a diagram illustrating a session key receiving method according to a third embodiment of the present invention;

Explanation of symbols

10 Transmission channel CP Content provider HN Home network HS Home server D1, D2, D3, D4 User equipment

Claims (14)

In a method for receiving a session key from a home server in a home network including a plurality of members,
a) Whenever a change occurs in the members of the home network, at least the session key and a session version representing the generation order of the session key are received, and at least the session key and the session version are set and stored in a table. The table includes a current session key table (CSKT) for storing the latest set and a past session key table (PSKT) for storing at least one set received so far. And storing the set stored in the current session key table in a past session key table, and storing the received set in the current session key table ;
b) receiving a license necessary for reproduction of the predetermined content;
c) Necessary for playback of the content by checking whether the same session version as the encrypted session version exists in the table based on the encrypted session version extracted from the license and the stored session version Determining a valid session key. The license includes a server identifier that is an identifier of the home server, and before the step c),
d) determining a session key necessary for reproducing the content by determining whether or not the server identifier included in the license is a server identifier of a home server to which a user device that receives the session key belongs. The method of claim 1 further comprising: The method of claim 2 , wherein the set further includes a server identifier of a home server that generated the session key . The license includes an encrypted content key generated by encrypting the content key using the server identifier, the session version, and the session key;
Step c)
4. The method of claim 3, wherein the method is performed by comparing a session version included in the license with a session version included in the current session key table. The license includes an encrypted content key generated by encrypting the content key using the server identifier, the session version, and the session key;
The step d)
4. The method of claim 3, wherein the method is performed by determining whether a server identifier included in the license is included in a server identifier included in the current session key table. The method according to claim 3, wherein the steps c) and d) are performed based on a past session key table (PSKT). Step c)
c1) if the session version from the license matches the current session version, determining the current session key as the session key;
c2) If the session version from the license is smaller than the current session version, check whether there is a session version from the license in the past session key table, and if it exists, the session version exists. The method of claim 6, further comprising: determining a past session key as the session key. The step d)
d1) The method according to claim 6, wherein if the server identifier from the license does not match the server identifier in the current session key table, the reproduction prohibition decision is made for the content. In a method in which a second user device transmits predetermined content from a first user device and reproduces the content in a home network including a plurality of user devices.
a) Every time a change occurs in the user equipment of the home network, at least a session key necessary for reproducing the content and a session version indicating the generation order of the session key are received, and at least the session key and the session version are received. Is stored in a table, and the table includes a current session key table (CSKT) for storing the latest set and a past session for storing at least one of the sets received so far. Storing the set stored in the current session key table in a past session key table and storing the received set in the current session key table, including a key table (PSKT) ;
b) receiving content encrypted from the first user device using a predetermined content key and a license necessary for playing the content;
c) Based on the encrypted session version included in the license and the stored session version, it is possible to reproduce the content by checking whether the same session version as the encrypted session version exists in the table. Determining the required session key;
d) generating the content key using the determined session key, and playing back the encrypted content using the generated content key. The license includes a server identifier that is an identifier of the home server, and before the step c),
e) determining whether the server identifier included in the license is the server identifier of the home server to which the user equipment receiving the session key belongs, thereby determining a session key necessary for reproducing the content. The method of claim 9 , further comprising a step. The method of claim 10 , wherein the set further includes a server identifier of a home server that generated the session key . The license includes an encrypted content key generated by encrypting the content key using the server identifier, the session version, and the session key;
Step c)
The method of claim 11 , wherein the method is performed by comparing a session version included in the license with a session version included in the current session key table. The license includes an encrypted content key generated by encrypting the content key using the server identifier, the session version, and the session key;
The step e)
The method of claim 11 , wherein the method is performed by determining whether a server identifier included in the license is included in a server identifier included in the current session key table. The method according to claim 11 , wherein the steps c) and e) are performed based on a past session key table (PSKT).

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